Thank you, it really compiles now  !

But the ugly thing is, I still see a lot of errors in the code-editor and the auto-complete doesn't work  . See the attachment please.

It looks like your plugin knows where to locate the headers when compiling, but VS doesn't have this information while in design-time.

Also, there is something wrong with the header-files also in other use-cases: I've tried to instantiate a String and got a compilation error:

Code:
String s = String(1);

Error:
GrilController.ino: 2:10: fatal error: string: No such file or directory
   #include <string>
   ^~~~~~~~
   compilation terminated

When I rewrite manually "#include <string>" to "#include <string.h>", it compiles, but the code stays red while editing. I'm not sure who inserts the "#include <string>" into my code automatically, if ReSharper or your plugin, but it's wrong.

Thank you

Wow, reinstalling Resharper did the last trick, you are a hero  !

Thank you so much & Greetz from Swiss!